1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device including a plurality of memory cells.
2. Description of the Background Art
Japanese Patent Application Laid-Open No. 2002-025286 discloses conventional nonvolatile semiconductor memory devices such as EPROM (Erasable Programmable ROM) and EEPROM (Electrically Erasable Programmable ROM). According to a nonvolatile semiconductor memory device disclosed in Japanese Patent Application Laid-Open No. 2002-025286, the difference between a signal from a certain memory cell and a predetermined reference signal is amplified by a differential sense amplifier, and the resultant output signal is considered as data written in the memory cell. Japanese Patent Application Laid-Open No. 2001-043691 also discloses a nonvolatile semiconductor memory device.
As described above, the differential sense amplifier employed in a conventional nonvolatile semiconductor memory device compares a signal from a memory cell with a predetermined reference signal. When a noise and the like generate under this configuration, it is difficult to secure a signal difference with enough amplitude at the input of the differential sense amplifier, thereby causing the differential sense amplifier to malfunction at times. As a result, reliability of the nonvolatile semiconductor memory device cannot be secured to a satisfactory degree in such a case.